1. Field of the Invention
The present invention relates to acoustic wave resonators utilizing surface acoustic waves or boundary acoustic waves. More particularly, the present invention relates to a one-port acoustic wave resonator in which apodization weighting is applied to IDT electrodes.
2. Description of the Related Art
Various resonators utilizing surface acoustic waves and boundary acoustic waves have been disclosed in the past. For example, Japanese Patent No. 2645674 indicated below discloses a surface acoustic wave resonator illustrated in FIG. 16 and FIG. 17. A surface acoustic wave resonator 501 illustrated in FIG. 16 is a surface acoustic wave resonator utilizing Love waves described in Japanese Patent No. 2645674 as an example of the related art. In this surface acoustic wave resonator 501, an interdigital transducer (IDT) electrode 503 is provided on a piezoelectric substrate 502. The IDT electrode 503 is a normal IDT electrode. The surface acoustic wave resonator 501 utilizes resonance characteristics based on Love waves, and there is a problem in that spurious responses caused by transverse modes are generated.
To address this, Japanese Patent No. 2645674 discloses a one-port surface acoustic wave resonator 511 having an electrode structure illustrated in FIG. 17 which suppresses spurious responses caused by transverse modes. In the surface acoustic wave resonator 511, reflectors 513 and 514 are arranged at opposite sides of an IDT electrode 512 in surface acoustic wave propagation directions. As illustrated in the FIG. 17, apodization weighting is applied to the IDT electrode 512 such that the cross width is increased in a center portion in the surface acoustic wave propagation directions and is reduced toward the ends in the surface acoustic wave propagation directions.
In the surface acoustic wave resonator 511, apodization weighting is applied to the IDT electrode 512 as described above, and, accordingly, spurious responses caused by transverse modes are suppressed to achieve satisfactory resonance characteristics.
As described in Japanese Patent No. 2645674, in a one-port surface acoustic wave resonator utilizing Love waves, it is possible to suppress transverse-mode spurious responses by applying apodization weighting to an IDT electrode as described above. However, there is a problem with the apodization weighting described in Japanese Patent No. 2645674 in that the center portion of the IDT electrode 512 has a very large cross width. Specifically, since the apodization weighting is applied to the IDT electrode 512 illustrated in FIG. 17, the cross width of electrode fingers is very large at the center in the surface acoustic wave propagation directions, as compared to the IDT electrode 503 illustrated in FIG. 16, where the areas of the crossing portions of the IDT electrodes are substantially the same. That is, the IDT electrode 512 has a large maximum cross width, and thus, has long electrode fingers, which results in a concentration of power consumption at the center of the IDT electrode 513. This decrease the power resistance.
In addition, because of the large maximum cross width, the IDT electrode 513 must have a relatively large dimension in the direction perpendicular to the surface acoustic wave propagation directions.
Furthermore, the surface acoustic wave resonator 511 has another problem in that the Q factor at an anti-resonant frequency is not sufficiently high.
The surface acoustic wave resonator 511 described in above utilizes Love waves. Love waves and Rayleigh waves have relatively low acoustic velocities and are likely to be confined in an IDT, which may result in the generation of a transverse mode. When using acoustic waves, such as Love waves and Rayleigh waves, which have relatively low acoustic velocities and are not leaky waves, it is difficult to suppress a transverse mode causing spurious responses by adjusting the crystal orientation of a piezoelectric substrate.